THE STEM. 



99 



air, not by their own riyiditv, but b)' the development ot 

 special members for < limbing purposes, such as recur\ ed 

 spines, tendrils, sensitive leaf stalks, or even by recurved 

 normal branches. (See •"^115, 158.) Others wrap them- 

 selves about objects of suitable size, and are called twining 

 stems. (See ■ 291.) The direction of twining varies with 

 different plants, but most commonly corresponds to the 

 movement of the hands of a watch, the support being sup- 

 posed to be in the center. 



124. Primary structure. — The origin of the stem-tissues 

 has already Ijeen described. (See ^ 100.) 



In following the stem from a[)ex to base it is readily 

 observed that the structure changes as the parts grow older. 

 It is possible, however, to select a point at which the stem in 



Fig. 



Fig. 



Fig. hi. — riiagram of a trans\'erse section of stem of Ih^ris avuit-a. sliu\\iQg outline, 

 and paired \'a&tular bundles. The black is the xyleni bundle : the gray is the phloem 

 bundle- The outer line represents the epidermis ; a circle including the bundles \MaiId 

 mark the limits ot the stele, with its central pith ; the cortex lies between the epidern.is 

 and stele. — After Nii'geli. 



Fig. 112. — Diagram of a transverse section of a jialm stem. The epidermis is represented 

 by the outer line : the endodermis by the inner one, with the narrow cortex between 

 them; the stele, with numerous bundles scattered through the pith, is within the 

 endodermis. — .After Frank. 



all cases attains a definite development. This [toint is at the 

 internode which has just reached its full length. The struc- 

 ture of the stem at this point may be designated as \\i primary 

 structure. If a thin section be cut from such an internode, 



